1. Field of the Invention
The present invention relates to an image sensing apparatus.
2. Description of the Related Art
There are conventional image sensing apparatuses such as digital cameras and digital video cameras that use a CCD image sensor or a CMOS image sensor as an image sensor. An image sensor is provided with a pixel array in which a plurality of pixels are arrayed in a direction along a row and in a direction along a column, a readout circuit that reads out signals from the pixel array via a plurality of column signal lines, and an output amplifier that outputs signals received via an output line from the readout circuit. With this image sensor, a gentle variation in the reference level of pixel signals (hereinafter, called “dark shading”) according to pixel position may arise, according to differences in the lengths of the column signal lines or output lines for each pixel signal. This dark shading can be corrected using signals output from pixels that are in a shaded state. This correction process is called dark shading correction.
The dark shading correction process is described in detail, using FIGS. 8A to 8D. In FIGS. 8A to 8D, the vertical axis shows the signal level, and the horizontal axis shows the position of the readout pixel in a direction along a column in the selected row.
Pixel signals of the levels shown in FIG. 8A that are output from the pixels to the column signal lines are also affected by the dark shading shown in FIG. 8C in the process of being transferred by the column signal lines and the output lines. The pixel signals sequentially received by the output amplifier via the output lines are, as shown in FIG. 8B, a signal obtaining by superimposing the dark shading component shown in FIG. 8C on the image signal component shown in FIG. 8A.
Here, as shown in FIG. 9, the readout circuit reads out black level reference signals from pixels in an entire pixel area AR3 or a partial area AR2 of a pixel array PA, in a state where the pixel array PA is shaded. Alternatively, the readout circuit reads out black level reference signals from a shaded area SA in which pixels in the pixel array PA are shaded (shown with diagonal lines). The readout circuit also reads out pixel signals that depend on the light, from pixels in an effective area EA. The image sensor outputs the black level reference signals and the pixel signals from the pixel array PA to the subsequent stage. Then, at the subsequent stage, the pixel signals are corrected using the black level reference signals.
For example, one-dimensional projection data (projection data in a direction along a column) is derived from signals of a prescribed area AR2 selected according to the purpose, and a moving average is taken in a lateral direction (direction along a row) and set as dark shading correction data (see FIG. 8D) in order to remove the effect of the noise component. By performing a correction process of subtracting this dark shading correction data from the pixel signals (FIG. 8B), true image signals (see FIG. 8A) from which the effect of dark shading has been removed can be obtained. Here, the above-mentioned moving average means a signal processing that involves successively grouping the signals of a fixed number of pixels and deriving average values.
The readout circuit often performs, after performing a readout operation of reading out the pixel signals from a selected row in the pixel array, a transfer operation of sequentially transferring the pixel signals of the selected column to an output line. Image signals can be obtained by performing the dark shading correction process on signals output from the image sensor after they have undergone the readout operation and the transfer operation. However, the period for which this readout operation and transfer operation are performed, that is, the total readout period, tends to become longer as the number of pixels included in the pixel array increases.
With the technique of Japanese Patent Laid-Open No. 2001-045375, signals for driving pixels are supplied to the pixels by a plurality of row control lines each of which extends in a direction along a row, and the signals of the pixels are read out via a plurality of column signal lines each of which extends in a direction along a column, in a pixel array in which a plurality of pixels are arrayed two-dimensionally. Two storage units are connected to one end of the column signal lines, and when the signals of one of the two storage units are being transferred to the subsequent stage, signals read out from pixels are stored in the other of the two storage units. The blanking period (period of no sensor output) can thereby be reduced, and the total readout period can be shortened.
Here, the two storage units are assumed to be a first storage unit (capacitances 14 and 13 in Japanese Patent Laid-Open No. 2001-045375) and a second storage unit (capacitances 12 and 11 in Japanese Patent Laid-Open No. 2001-045375). In this case, the readout circuit, for the period that the pixel signals of a prescribed row (e.g., (n−1)th row) are being transferred from the second storage unit to an output line, turns on a readout switch in response to a driving pulse, and reads out the pixel signals of the next row to be read out (e.g., (n)th row) to the first storage unit. Note that n is an integer of 2 or more.
In this case, during the period that the signals of the (n−1)th row are transferred to the output lines, the power supply of the readout circuit and the voltage of the signal lines fluctuate due to the driving pulse for reading out the signals of the (n)th row to the first storage unit. Following this, the effect of the transition in the level of the driving pulse appears as noise in the transferred pixel signals of the (n−1)th row. Since this noise appears at a comparable level in the same column address in the pixel signals of each row that are transferred, it appears as linear noise in a vertical direction (=direction along a column) when viewed as an image.
Here, with conventional dark shading correction, correction is performed by storing a certain fixed value and by subtracting this fixed value from actually-captured image. However, when actually capturing an image of an object, since the brightness of the object is not known beforehand and therefore the brightness of the pixels to be corrected is not known beforehand, it is difficult to know the amount of noise to subtract from the actually-captured image beforehand. Thus, the possibility exists of not being able to sufficiently correct the noise simply by storing and subtracting a fixed value.